The present invention relates to the high temperature growth of large single crystals, and in particular relates to methods and apparatus for the growth of high-quality single crystals of silicon carbide.
Silicon carbide is a perennial candidate for use as a semiconductor material. Silicon carbide has a wide bandgap, a low dielectric constant, and is stable at temperatures far higher than temperatures at which other semiconductor materials, such as silicon, become unstable. These and other characteristics give silicon carbide excellent semiconducting properties. Electronic devices made from silicon carbide can be expected to perform, inter alia, at higher temperatures, faster speeds and at higher radiation densities, than devices made from other commonly used semiconductor materials such as silicon.
Those familiar with solid-state physics and the behavior of semiconductors know that a semiconductor material must have certain characteristics to be useful as a material from which electrical devices may be manufactured. In many applications, a single crystal is required, with low levels of defects in the crystal lattice, along with low levels of unwanted chemical and physical impurities. If the impurities cannot be controlled, the material is generally unsatisfactory for use in electrical devices. Even in a pure material, a defective lattice structure can prevent the material from being useful.
Silicon carbide possesses other desirable physical characteristics in addition to its electrical properties. It is very hard, possessing a hardness of 8.5-9.25 Mohs depending on the polytype [i.e., atomic arrangement] and crystallographic direction. In comparison, diamond possesses a hardness of 10 Mohs. Silicon carbide is brilliant, possessing a refractive index of 2.5-2.71 depending on the polytype. In comparison, diamond""s refractive index is approximately 2.4. Furthermore, silicon carbide is a tough and extremely stable material that can be heated to more than 2000xc2x0 C. in air without suffering damage. These physical characteristics make silicon carbide an ideal substitute for naturally occurring gemstones. The use of silicon carbide as gemstones is described in U.S Pat. Nos. 5,723,391 and 5,762,896 to Hunter et al.
Accordingly, and because the physical characteristics and potential uses for silicon carbide have been recognized for some time, a number of researchers have suggested a number of techniques for forming crystals of silicon carbide. These techniques generally fall into two broad categories, although it will be understood that some techniques are not necessarily so easily classified. The first technique is known as chemical vapor deposition (CVD) in which reactants and gases are introduced into a system within which they form silicon carbide crystals upon an appropriate substrate.
The other main technique for growing silicon carbide crystals is generally referred to as the sublimation technique. As the designation xe2x80x9csublimationxe2x80x9d implies, sublimation techniques generally use some kind of solid silicon carbide starting material, which is heated until the solid silicon carbide sublimes. The vaporized silicon carbide starting material is then encouraged to condense on a substrate, such as a seed crystal, with the condensation intended to produce the desired crystal polytype.
One of the first sublimation techniques of any practical usefulness for producing better crystals was developed in the 1950s by J. A. Lely, and is described in U.S. Pat. No. 2,854,364. From a general standpoint, Lely""s technique lines the interior of a carbon vessel with a silicon carbide source material. By heating the vessel to a temperature at which silicon carbide sublimes, and then allowing it to condense, re-crystallized silicon carbide is encouraged to deposit along the lining of the vessel.
The Lely sublimation technique was modified and improved upon by several researchers. Hergenrother, U.S. Pat. No. 3,228,756 (xe2x80x9cHergenrother ""756xe2x80x9d) discusses another sublimation growth technique, which utilizes a seed crystal of silicon carbide upon which other silicon carbide condenses to grow a crystal. Hergenrother ""756 suggests that in order to promote proper growth, the seed crystal must be heated to an appropriate temperature, generally over 2000xc2x0 C. in such a manner that the time period during which the seed crystal is at temperatures between 1800xc2x0 C. and 2000xc2x0 C. is minimized.
Ozarow, U.S. Pat. No. 3,236,780 (xe2x80x9cOzarow ""780xe2x80x9d) discusses another unseeded sublimation technique which utilizes a lining of silicon carbide within a carbon vessel. Ozarow ""780 attempts to establish a radial temperature gradient between the silicon carbide lined inner portion of the vessel and the outer portion of the vessel.
Knippenberg, U.S. Pat. No. 3,615,930 (xe2x80x9cKnippenberg ""930xe2x80x9d) and U.S. Pat. No. 3,962,406 (xe2x80x9cKnippenberg ""406xe2x80x9d) discuss alternative methods for growing silicon carbide in a desired fashion. The Knippenberg ""930 patent discusses a method of growing p-n junctions in silicon carbide as a crystal grows by sublimation. According to the discussion in this patent, silicon carbide is heated in an enclosed space in the presence of an inert gas containing a donor type dopant atom. The dopant material is then evacuated from the vessel and the vessel is reheated in the presence of an acceptor dopant. This technique is intended to result in adjacent crystal portions having opposite conductivity types thereby forming a p-n junction.
The Knippenberg ""406 patent discusses a three-step process for forming silicon carbide in which a silicon dioxide core is packed entirely within a surrounding mass of either granular silicon carbide or materials that will form silicon carbide. The packed mass of silicon carbide and silicon dioxide is then heated. The system is heated to a temperature at which a silicon carbide shell forms around the silicon dioxide core, and then further heated to vaporize the silicon dioxide from within the silicon carbide shell. Finally, the system is heated even further to encourage additional silicon carbide to continue to grow within the silicon carbide shell.
Vodakov, U.S. Pat. No. 4,147,572 discusses a geometry oriented sublimation technique in which solid silicon carbide source material and seed crystals are arranged in a parallel close proximity relationship to another.
Addamiano, U.S. Pat. No. 4,556,436 (xe2x80x9cAddamiano ""436xe2x80x9d) discusses a Lely-type furnace system for forming thin films of beta silicon carbide on alpha silicon carbide which is characterized by a rapid cooling from sublimation temperatures of between 2300xc2x0 C. and 2700xc2x0 C. to another temperature of less than 1800xc2x0 C. Addamiano ""436 notes that large single crystals of cubic (beta) silicon carbide are simply not available and that growth of silicon carbide or other materials such as silicon or diamond is rather difficult.
Hsu, U.S. Pat. No. 4,664,944, discusses a fluidized bed technique for forming silicon carbide crystals which resembles a chemical vapor deposition technique in its use of non-silicon carbide reactants, but which includes silicon carbide particles in the fluidized bed, thus somewhat resembling the sublimation technique.
German (Federal Republic) Patent No. 3,230,727 to Siemens Corporation discusses a silicon carbide sublimation technique in which the emphasis of the discussion is the minimization of the thermal gradient between a silicon carbide seed crystal and silicon carbide source material. This patent suggests limiting the thermal gradient to no more than 20xc2x0 C. per centimeter of distance between source and seed in the reaction vessel. This patent also suggests that the overall vapor pressure in the sublimation system be kept in the range of between 1 and 5 millibar and preferably around 1.5 to 2.5 millibar.
Davis, U.S. Pat. No. Re. 34,861 (xe2x80x9cDavis ""861xe2x80x9d) discuss a method of forming large device quality single crystals of silicon carbide. This patent presents a sublimation process enhanced by maintaining a constant polytype composition and size distribution in the source materials. These patents also discuss specific preparation of the growth surface and seed crystals and controlling the thermal gradient between the source materials and the seed crystal.
Barrett, U.S. Pat. No. 5,746,827 (xe2x80x9cBarrett ""827xe2x80x9d) discusses a method for producing large diameter silicon carbide crystals requiring two growth stages. The first growth stage is to isothermally grow a seed crystal to a larger diameter. The second growth stage is to grow a large diameter boule from the seed crystal under thermal gradient conditions.
Hopkins, U.S. Pat. No. 5,873,937 (xe2x80x9cHopkins ""937xe2x80x9d) discusses a method for growing 4H silicon carbide crystals. This patent teaches a physical vapor transport (PVT) system where the surface temperature of the crystal is maintained at less than about 2160xc2x0 C. and the pressure inside the PVT system is decreased to compensate for the lower growth temperature.
Kitoh, U.S. Pat. No. 5,895,526 (xe2x80x9cKitoh ""526xe2x80x9d) teaches a sublimation process for growing a single silicon carbide crystal where the sublimed source material flows parallel with the surface of a single crystal substrate.
Although significant progress in the production of SiC crystals has occurred over the years, commercially significant goals still remain for SiC crystal production. For example, faster and more powerful prototype devices are being developed that require larger SiC crystals that maintain or improve upon current crystal quality. Boules large enough to produce 50-mm diameter SiC wafers are currently at the far end of commercially viable SiC production. 75-mm diameter wafers of good quality have been demonstrated but are not yet commercially available and there is already a need for 100-mm wafers. Many SiC crystal production techniques are simply incapable of economically and consistently producing crystals of the size and quality needed. The primary reason for the inability of most crystal production techniques to keep up with commercial demand lies within the chemistry of SiC.
The chemistry of silicon carbide sublimation and crystallization is such that the known methods of growing silicon carbide crystals are difficult, even when carried out successfully. The stoichiometry of the crystal growth process is critical and complicated. Too much or too little silicon or carbon in the sublimed vapor may result in a crystal having an undesired polytype or imperfections such as micropipes.
Likewise, the high operating temperatures, typically above 2100xc2x0 C. and the necessity of forming specific temperature gradients within the crystal growth system pose significant operational difficulties. The traditional graphite sublimation containers utilized in most sublimation systems possess infrared emissivities on the order of 0.85 to 0.95 depending upon the container""s surface characteristics. Seed crystals are heat sensitive to infrared radiation. Therefore, the infrared radiation emitted by the graphite containers can overheat the seed crystal thereby complicating the precise temperature gradients necessary for successful operation of sublimation systems.
Recently, the SiC group at Linkxc3x6ping University presented a technique for the growth of SiC called High Temperature Chemical Vapor Deposition (xe2x80x9cHTCVDxe2x80x9d). O. Kordina, et al., xe2x80x9cHigh Temperature Chemical Vapor Deposition,xe2x80x9d paper presented at the International Conference on SiC and Related Materials, Kyoto, Japan, 1995; See also O. Kordina, et al., 69 Applied Physics Letters, 1456 (1996). In this technique, the solid silicon source material is replaced by gases such as silane. The use of gaseous source materials improves control of the reaction stoichiometry. The solid carbon source material may also be replaced by a gas such as propane; however, most of the carbon utilized in this technique actually comes from the graphite walls of the crucible. Theoretically, this technique""s utilization of a continuous supply of gas would allow continuous and extended SiC boule growth. Unfortunately, the HTCVD technique has not proven commercially useful for boule growth primarily because the reaction destroys the graphite crucibles used in the process. Furthermore, the addition of hydrocarbon gases in this particular process tends to produce Si droplets encrusted with SiC which decreases efficiency and also ties up Si and C thereby altering the stoichiometry of the system.
Perhaps the most difficult aspect of silicon carbide growth is the reactivity of silicon at high temperatures. Silicon reacts with the graphite containers utilized in most sublimation processes and, as noted above, is encouraged to do so in some applications. This reaction is difficult to control and usually results in too much silicon or too much carbon being present in the system thus undesirably altering the stoichiometry of the crystal growth process. In addition, silicon""s attack on the graphite container pits the walls of the container destroying the container and forming carbon dust which contaminates the crystal.
In attempts to resolve these problems, some research has evaluated that the presence of tantalum in a sublimation system, e.g., Yu. A. Vodakov et al, xe2x80x9cThe Use of Tantalum Container Material for Quality Improvement of SiC Crystals Grown by the Sublimation Technique,xe2x80x9d presented at the 6th International Conference on Silicon Carbide, September 1995, Kyoto, Japan. Some researchers opine that the presence of tantalum helps maintain the required stoichiometry for optimal crystal growth. Such an opinion is supported by reports that sublimation containers comprising tantalum are less susceptible to attack by reactive silicon.
In a related application, WO97/27350 (xe2x80x9cVodakov ""350xe2x80x9d) Vodakov presents a sublimation technique similar to that presented in U.S. Pat. No. 4,147,572 and attempts to address the problem of silicon attacking the structural components of the sublimation system. Vodakov ""350 describes a geometry oriented sublimation technique in which solid silicon carbide source materials and seed crystals are arranged in parallel close proximity relationship to another. Vodakov ""350 utilizes a sublimation container made of solid tantalum. The inner surface of Vodakov""s tantalum container is described as being an alloy of tantalum, silicon and carbon. Page 11, line 26 through page12, line 10. Vodakov claims that such a container is resistive to attack by silicon vapor and contributes to well-formed silicon carbide crystals.
The cost of tantalum is, however, a drawback to a sublimation process utilizing the container described in Vodakov. A sublimation container of solid tantalum is extremely expensive and like all sublimation containers, will eventually fail, making its long-term use uneconomic. A solid tantalum sublimation container is also difficult to machine. Physically forming such a container is not an easy task. Lastly, the sublimation process of Vodakov ""350 suffers the same deficiency shown in other solid source sublimation techniques in that it is not efficient at forming the large, high quality boules needed for newly discovered applications.
Therefore, a need exists for a process that provides for controlled, extended and repeatable growth of high quality SiC crystals. Such a system must necessarily provide a container that is resistive to attack by silicon. Such a system should also be economical to implement and use.
Accordingly, an object of the present invention is to provide a method and apparatus for the controlled, extended and repeatable growth of high quality silicon carbide crystals of a desired polytype.
A further object of the present invention is to provide a method of growing high. quality single crystals of silicon carbide by controlling the stoichiometry of the crystal growth process.
A further object of the present invention is to provide a method of growing high quality single crystals of silicon carbide by controlling the temperature of the crystal growth process.
A further object of the present invention is to provide a method and apparatus for growing high quality single crystals of silicon carbide by reducing or eliminating impurities resulting from degradation of the physical components of the system.
A still further object of the present invention is to provide for a system for SiC crystal growth that resists reaction with vaporized silicon.
The invention meets these objects with a method and apparatus for growing large single crystals of SiC for use in producing electrical devices and for use as gemstones. In particular, the invention encompasses introducing a monocrystalline seed crystal of SiC of a desired polytype and a source of silicon and a source of carbon into SiC crystal growth system typically comprising a crucible and a furnace. The source of silicon and carbon is then raised to a temperature sufficient for the formation of vaporized species containing silicon and carbon. The temperature of the seed crystal is raised to a temperature approaching but lower than the temperature of the silicon and carbon vapors and lower than that at which SiC will sublime faster than deposit under the gas pressure conditions within the crucible, thus creating a temperature gradient within the crucible.
A suitable flow of a vaporized species containing silicon and carbon derived from the source of silicon and the source of carbon is generated and maintained within the crucible. The flow of vapor is directed to the growth surface of the seed crystal for a time sufficient to produce a desired amount of macroscopic growth of monocrystalline SiC while substantially preventing any silicon containing species from reacting with material utilized in constructing the SiC crystal growth system.
The foregoing and other objects, advantages and features of the invention, and the manner in which the same is accomplished will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments and wherein: